We estimated total lake plant biomass and primary net production in two shallow Swedish lakes that differ in nutrient loading and plant form dominance. In clearwater Lake Krankesjö n (10 mg chlorophyll a L 21 ), submerged macrophytes contributed more than phytoplankton and epiphyton to the estimated plant biomass. Estimated net primary production during May to September was 90-130, 1.2, and 14 g C m 22 for phytoplankton, epiphyton, and submerged macrophytes, respectively. In turbid Lake Bö rringesjö n (60-80 mg chlorophyll a L 21 ), primary production by submerged macrophytes and periphyton was negligible. Although gross primary production of phytoplankton was high close to the water surface, estimated areal net primary production during May to September was low, 240 to +25 g C m 22 , as a result of self-shading and high respiration. Grazing pressure from zooplankton rarely exceeded 15% d 21 in both lakes, indicating that phytoplankton production was not limited by grazing. Low gross epiphyton production could result from high grazing by macroinvertebrates and thus higher trophic transfer efficiency through the benthic than through the pelagic food web. Provided that conditions in Lake Bö rringesjö n reflect previous turbid state conditions in Lake Krankesjö n, our results may explain why a shift to a clearwater state was followed by increased biomass of higher trophic levels. Our results also support the paradox of enrichment hypothesis, which predicts lower productivity at high nutrient loading. Contrary to former investigations, we found lower production at a higher nutrient loading already at the trophic level of primary producers.

Primary production of undisturbed sediments, taken from 2 shallow coastal lagoons in the southern Baltic Sea, was measured in situ and in the laboratory. In the highly eutrophicated and turbid water (mean attenuation coefficient = 2.97 m-') of Kirr-Bucht (Darss-Zingster Bodden), chlorophyll a concentrations were about 8 times higher than at moderately eutrophicated (mean attenuation coefficient = 0.66 m-') Rassower Strom (Nordrugensche Bodden). The difference between the 2 areas in terms of chlorophyll a concentrations of the sediment was smaller (only about 2.5 times higher at f i rBucht). Sediment oxygen consumption rates were slmilar at both sampling stations, showing a marked seasonal trend. Hourly rates of gross production, measured by in situ incubations, revealed maximal values of 30.0 and 17.6 mg C m-2 h-' for sediments of Kirr-Bucht and Rassower Strom, respectively. Agreement between production rates measured in situ and in laboratory incubations was acceptable (R2 = 0.833). Irradiance dependency of gross production could be well described by 2 parameters, a (slope of light-saturation curve at low irradiance) and P, (photosynthetic capacity), of a photosynthesis model. Calculated annual rates for Kirr-Bucht (0.6 m water depth) were 60 g C m-2 yr-' for gross production and 5 g C m-' y r ' for net production. For Rassower Strom (3.4 m water depth). 23 g C m-' yr-' for gross production and -31 g C m-2 yr-' for net production were calculated. If production rates were extrapolated to irradiance conditions at comparable water depths at both sampling locations (e.g. 1 m), the detrunental effect of the eutrophicated water column of Kirr-Bucht on benthic primary production became evident in the gross production rates (Irr-Bucht: 25 g C m-2 y f ', Rassower Strom: 51 g C m-2 yr-l) and was even more significant for the net production rates (Kirr-Bucht: -30 g C m-' yr-', Rassower Strom: -3 g C m-2 yr-'). At a depth of 1 m, a much higher deficit in the carbon balance for the benthic system of firr-Bucht was calculated than for Rassower Strom. Therefore, benthic community respiration at the highly eutrophicated location could not by a large margin be balanced by authochthonous benthic primary production, whereas, at the moderately eutrophlcated location, benthic respiration and production were nearly balanced.

Primary production by pelagic and benthic rnicroalgae was measured in 2 shallow coastal lagoons in the southern Baltlc Sea by in situ and laboratory incubations to quantify the significance of these 2 malor groups of primary producers in absolute and relative terms. Pelagc primary production rates per volume at saturating hght conditions were 7 to 12 times higher at the polytrophic location KirrBucht (Darss-Zingst Bodden) than at the meso-/eutrophic location Rassower Strom (Nordriigensche Bodden). Chlorophyll a specific primary production rates, however, were in the same range at both locations, except for 1 situation in spnng which is exphcitly discussed. The pelagic chlorophyll a specific maxin~unl light utilization coefficient ( a ' ) tended towards higher values in Kirr-Bucht samples, which can be interpreted as an adaptation to a lower supply of irradiance. Pelagic and benthic primary production varied seasonally, with highest rates occurring in June/July. A calculation of short-term variability due to cloud-modulated avadabllitv of irradiance revealed reductions of pelaqic and benthic gross production (GP) rates on overcast days, compared to bright days, in the range of 26 to 69% and 62 to 82 "C,, respectively. Net community production (NP) rates were more severely reduced than GP rates on overcast days. Benthic NP was generally reduced to below zero on overcast days, whereas it was nlostly positive on bright days at both locations. To evaluate the significance of the different optical properties of both types of water (diffuse vertical attenuation coefficient, k = 3.3 to 3.7 and 0.5 to 0.7 m-' at Kirr-Bucht and Rassower Strom, respectively) for pnrnary production, the benthic and pelagic contributions at both sites were calculated regarding a water depth of 2 m, which is about the mean depth of the Bodden systems This standardization revealed obvious differences in basic conditions for primary production which remained undetected when the comparison of the 2 sites was based on the water depths in which the in situ incubations had been carried out (0.6 and 3.4 m at Kirr-Bucht and Rassower Strom, respectively) In meso-/eutrophic Rassower Strom at a depth of 2 m, benthic GP would contribute about 30 to 45% to the total GP of microalgae, depending on weather conditions and seasons. At the same depth in polytrophic Kirr-Bucht, the microalgae on the sediment surface would account for only 0 3 % or less of total GP. This demonstrates that optical properties of the water, which are closely related to the trophic state of the system, exert strong control on varying proportions of productivity, attributable to planktonic versus benthic nlicroalgal communities. It can be concluded from the modeled data that the difference between both locations will be most prominent dunng periods of low supply of radlant energy, Like on overcast days.

No abstract

Eutrophication in coastal areas has stimulated phytoplankton growth, sustaining a high biomass and leading to a shift in the underwater light field. With the significance of the microphytobenthos for oxygen supply and carbon budget of both benthic and pelagic habitats in mind, the possible effects of reduced light availability were investigated in the estuarine Bodden area (southern Baltic Sea) at 2 sites differing in trophic status -the eutrophic Kirr Bucht (KB) and the mesotrophic Rassower Strom (RS). Using for the first time microsensors in Bodden sediments, it was possible to visualize small-scale heterogeneity in the light regime, photosynthetic activity and oxygen penetration with high spatial and temporal resolution. Hence, differences at the 2 sites related to sediment characteristics (KB sandy, RS muddy), and photoautotrophic biomass (benthic chlorophyll a in the upper 1 cm, µg cm -3 = 11 to 48 at KB and 13 to 17 at RS) could be ruled out. Calculations of benthic primary production based solely on microelectrode measurements revealed substantial oxygen fluxes and carbon fixation rates at in situ light intensities at both study sites (e.g. gross primary production, GPP, mg C m -2 h -1 = 28 to 80 at KB and 3 to 36 at RS). The different combinations of water transparency (pelagic chlorophyll a, µg l -1 = 12 to 33 at KB and 1.3 to 4.5 at RS), light attenuation k (3.17 m -1 at KB, 0.61 m -1 at RS) and water depth (0.6 m at KB, 3.4 m at RA) have led to a similar light availability for benthic algae on the sediment surface at both study sites. Consequently, the benthic algae had comparable productivity at both sites, with maximum primary production, P B max (mg C mg -1 chlorophyll a h -1 ) of 0.29 to 1.46 at KB and 0.17 to 1.63 at RS; and were adapted to rather low light conditions, with light saturation, E k (µE m -2 s -1 ) of 22 to 152 at KB and 10 to 116 at RS). Varying with season, microphytobenthic photosynthetic activity accounted for 26 to 59 and 2 to 53% to the total primary production at the KB and RS, respectively, with the highest contribution in spring coincident with the most favourable light conditions at the sediment surface. With an annual average of about 37 and 30% (KB and RS, respectively), the contribution of the microphytobenthos to total production was significant and comparable at both study sites. Nevertheless, the higher trophic status at KB resulted in a change in the benthic microalgal community towards sedimentated phytoplankton species and had a negative impact on microphytobenthic primary production rates. This was estimated by calculating the gross primary production for varying water depths on the basis of the different water transparency at the 2 sites.